1. Field of the Invention
The invention relates to radars.
2. Discussion of Prior Art
Intrinsically, the reception part of a radar provides a representation of the electromagnetic energy received, within a frequency band determined by its emission characteristics, having regard to the application concerned. Reference is frequently made to radar "echoes", but, in order to define such echoes, it is already necessary to carry out a processing utilizing the energy received.
Conventionally, such a processing includes the analysis of the energy received in the time-frequency domain. In principle, the time analysis involves range "ports"(or "windows"), while the frequency analysis involves Doppler ports (or windows), whilst certain radars have a more subtle operation (for example, the case of a linear frequency modulation on emission). The combination of the breakdown into range ports on the one hand and Doppler ports on the other hand provides range-Doppler diagrams.
Stricto sensu, there is a "radar echo" only if the energy received in a given plot exceeds a certain threshold. The definition of such thresholds is important, since it determines the "false-alarm probability" of the radar.
The designation "false-alarm control" is used to refer to the means permitting the definition of the said thresholds, or alternatively criteria which are more sophisticated than a simple threshold and permit the discrimination of echoes in the energy received. If this control is badly performed, the system for the exploitation of the radar echoes will have, in principle, too many plots detected to be analyzed, and will be saturated (it might also have too few of these, which would signify that objects to be detected will be missed, which is an absolutely unacceptable situation).
In order to discriminate the genuine echoes, it is necessary to eliminate a priori the thermal noise. In numerous radars, the fixed objects ("ground echoes") are also to be eliminated. A difficulty arises in these circumstances, since, if the thermal noise is of a very low level, conversely the ground echoes are of a high level, or even a very high level, that is to say greater than the level of the useful echoes.
This difficulty is partially alleviated by the fact that, in general, means are available for specifically filtering the ground echoes. Thus, the problem exists only in the case of those of the ground echoes which have escaped the filtering. The origin of these "parasitic residues" differs, depending upon whether the radar is fixed or carried by a movable platform, such as an aircraft.
In the case of a fixed radar, the ground echoes of close origin may exhibit very large radar equivalent surfaces. This is so, for example, in the case of hangars or other constructions close to a radar for monitoring air traffic, and reflecting the noise energy of the emitter. The false-alarm control may be undertaken:
either by placing a limiter at the input of the Doppler filter, to desensitize the receiver in the presence of a strong echo, PA1 or by slaving the detection threshold as a function of the level of the echo measured before Doppler filtering; this second technique does not modify the spectrum of the signals passing through the Doppler filter, and can therefore be used even if the spectrum of the ground echoes is relatively extended in frequency.
In the case of a movable radar, the problem of false-alarm control is radically different, since the origin of these false alarms is entirely different.
First of all, the ground echoes at very high level are rare, since in principle the main lobe of the antenna does not illuminate the ground at short range. It remains the case that the side lobes of the antenna (if they are large) may receive ground echoes of relatively high level, with a Doppler frequency which corresponds to that of useful echoes. The usual means for controlling the false alarms consists in utilizing two channels, that is to say two receivers associated with two antennae; the first channel operates normally, while the second, of omnidirectional type, or more precisely of less directional type, is controlled in such a manner as to cover the side lobes of the first channel. The false-alarm control is then undertaken by comparing the outputs of the two channels for each range/Doppler resolution cell or plot. A useful signal corresponds to the case where the output level of the first channel exceeds that of the second, for the same plot.
This means does not give complete satisfaction in all cases. In particular, there are applications in which it is impossible to provide a second channel having the characteristics of "less directionality" which are desired. It is, in fact, necessary that the radiation pattern of this second channel should be situated significantly below that of the first channel, in the main lobe of the latter, and slightly above that of the first channel, in its side lobes; failing this, the false-alarm control might involve losses of detection with respect to a useful target, since the detection threshold would become abnormally high as soon as ground echoes, even of low level, appeared in the auxiliary channel, which ground echoes would not correspond to the side lobes of the first channel.
This problem arises, in particular, when the main lobe does not have a very high gain (aerial at large angle), and when the level of the side lobes is not well known or is difficult to determine with a sufficient precision.